In conventional planographic offset printing, it is known that different areas of a printing plate can be made to function as image areas and non-image areas by image-wise formation of different surface-chemical properties (oleophilic and hydrophilic properties) on the surface of the printing plate, so that dampening water fed to the non-image area forms a water film and acts as an ink repellent layer. Such planographic offset printing plates are the most widely used at present because they have the advantage that the plate-making is relatively easy and requires low cost, and the printed matters produced therewith have high quality. However, the need to use dampening water causes the following problems:
(a) Adjustment of the proper ink-water balance at a press operation is difficult and requires skill, which impedes labor-saving and automation.
(b) When printing is started or re-started after a rest interval, the ink-water balance does not become stable until a large number of sheets have been printed, so the spoilage rate is high.
(c) The printing paper undergoes expansion due to the presence of water, which is apt to cause bad registration.
(d) Due to the emulsification of the ink by dampening water, the ink adherability to the printing plate becomes poor, which creates problems both in dot reproduction and in color reproduction.
(e) The printing press must be provided with a dampening system, so it becomes large in size and high in cost.
In the hope of finding fundamental solutions to these problems, various studies have been under way for more than ten years on dry planographic printing plates using a silicone rubber layer as an ink repellent layer, Japanese Patent Publication No. 23042/1969 described for the first time the use of silicone rubber as an ink repellent layer. Since then, various types of printing plates have been proposed in, for example, U.S. Pat. No. 3,511,178, GB No. 1,399,949, GB No. 1,419,643, U.S. Pat. No. 4,358,522, GB No. 2,064,803, Japanese Patent Laid-Open Application Nos. 50102/1975 and 80047/1981. These dry planographic printing plates in which a silicone rubber layer overlays a photosensitive layer fall generally into two types: one type prepared by image-wise exposure of a presensitized plate, followed by treatment with a developer to dissolve either exposed or unexposed area of the underlying photosensitive layer and remove it together with an overlying silicone rubber layer leaving an image, and another type prepared by image-wise exposure of a presensitized plate using a photoadhesive or photoreleasable photosensitive layer, followed by development of the exposed plate so as to swell and remove only the silicone rubber layer in the unexposed area on the photoadhesive photosensitive layer or in the exposed area on the photoreleasable photosensitive layer, leaving an image.
In the former type, even if the adhesive strength between the silicone rubber layer and the photosensitive layer of the presensitized plate is designed to be very high, it is very rare that the image forming property thereof is poor or that the presensitzed plate cannot be sufficiently developed because the photosensitive layer is dissolved in the developer and is removed together with the overlaying silicone rubber layer. Therefore, it is possible to make a presensitized plate which has no protective film on the silicone rubber layer and a printing plate prepared from such a presensitized plate has stable adhesive strength of the silicone rubber, scratch-resistance, rub-resistance and high durability in printing.
In the latter type, in contrast, although it is possible to put a thinner ink layer onto the painting plate, it is impossible to develop the presensitized plate if the adhesive strength between the silicone rubber layer and the photosensitive layer is too high. Thus, it is difficult to make a dry planographic printing plate of the latter type which has sufficient scratch-resistance and rub-resistance.
When a dry presensitized plate comprising a base substrate, a photosensitive layer and a silicone rubber layer which are laminated one upon another in this order is to be made a negative working plate, it is necessary to use a photosolubilizable one as the photosensitive layer. This type of dry presensitized plate is disclosed in U.S. Pat. No. 3,511,178. When this plate is imagewise exposed to light, the photosensitive layer in the exposed area undergoes photolysis, becoming soluble in the developer, and is removed together with the overlay ing silicone rubber layer, so that the exposed area is now an image area. On the other hand, the photosolubilizable layer of a non-image or unexposed area still remains photosensitive even after the imagewise exposure, and therefore it becomes soluble when it is exposed to sunlight or light from a fluorescent tube.
There is a possibility that a printing plate may come in contact with an organic solvent similar to the solvent used for development, as in the case of treating with a plate-cleaner or applying a printing ink thereon during the press operation. Even if an excellent image has been reproduced on a printing plate after developing, a photosensitive layer of a non-image area will be removed together with the overlaying silicone rubber layer if the photosensitive layer of the non-image area comes in contact with an organic solvent to become soluble therein, so that the excellent image on the plate will be destroyed.
To solve the aforementioned problem, Japanese Patent Laid-Open Application Nos. 60947/1980 and 192956/1982 have proposed a method of fixing an image on a dry planographic printing plate by treating it with heat after exposing and developing a dry negative working presensitized plate. The process has disadvantages in that it requires a heat-treating system and that the image area cannot be sufficiently fixed so that it becomes soluble in an organic solvent when it is exposed to light after the treatment, which leads to a change in the tone of the dot image. Japanese Patent Laid-Open Application No. 205740/1982 has disclosed a method of fixing an image by treating it with a basic compound after the exposure and subsequent development. This process can be successfully applied to a dry negative working presensitized plate comprising a base substrate, a photoreleasable photosensitive layer and a silicone rubber layer coated on the substrate in this order wherein only the silicone rubber layer photoexfoliated is swelled and removed as shown in Example 3 of the reference. However, this process cannot be successfully applied to a dry presensitized plate wherein a photosensitive layer thereof is dissolved in the developer and removed together with the overlaying silicone rubber layer, as shown in Examples 1 and 2 of the reference.